This invention pertains to the field of measuring and indicating vehicular speed, particularly the speed of wheeled vehicles. Within that field it is related to digital speedometers that sense speed by timing a train of electrical pulses that are generated in proportion to wheel rotation. Most specifically, this invention relates to the class of such speedometers in which speed readings are retained for appropriate periods of time, rather than being continually lost as soon as each new speed sample is measured, and in which the retained readings may be recalled and displayed without the aid of any external apparatus. The applicable art in this field includes tachometers, for a device that measures shaft rotation rate can be calibrated to measure the linear speed of a wheeled vehicle.
The invention is also related to tachographs, but differs from known tachographs in that it requires no consumable or removable recording medium, such as paper or magnetic tape, and requires no medium-driving motor.
The prior art provides examples of digital speedometers and tachometers that may be adapted to serve as elements in the present invention. They have in common the inclusion of a digital register that contains the most recently sampled speed. In some, the sampling, which is to say the loading or latching of the register, is accomplished synchronously and at a fixed rate (e.g. Webster, Sugiyama, and Powell). Eshraghian and Summer sample synchronously, the former at a rate determined by rate of change of speed, and the latter at a rate slightly dependent upon speed. Dixon et al sample asynchronously at a rate proportional to speed. In Sugiyama one sample reading is retained and displayed.